1. Field of the Invention
This invention relates to a diaphragm aperture control device in a camera for automatically controlling the aperture value of an objective lens in accordance with the brightness of the object to be photographed, the shutter speed and the film sensitivity.
2. Description of the Prior Art
This camera is of the type which has an EE function of the shutter speed first and in which the object light passing through the diaphragm aperture is measured while the diaphragm aperture is stopped down from its open position prior to the commencement of exposure in association with a shutter releasing operation, and the stop down operation is stopped to control the aperture to a proper value when the metering output has reached a predetermined value.
However, when the stop down operation of the diaphragm aperture is stopped by a locking member, there is a time delay in the operation of an electromagnetic mechanism for operating the locking member by reason of the fact that the metering output has reached a predetermined value and a delay in the operation of the mechanical part including the locking member, so that there is a disadvantage that the aperture is stopped down more than a proper value by the time delay .DELTA.t.
Therefore, in order properly to control the aperture value, in a device wherein, for example, an electromagnet is demagnetized to drive the locking member, the time whereat the magnet is demagnetized may be earlier by the delay time .DELTA.t. In such a device, the magnet is demagnetized by previously examining a reference level whereat the magnet is to be demagnetized from the metering output characteristic and when the metering output has assumed that reference level. However, the reference level is not constant depending on the speed at which the aperture is stopped down, namely, the speed of variation of the metering output and accordingly, the time from the commencement of the stop down of the aperture till the demagnetization of the electromagnet cannot be accurately determined.
As a solution to this problem, there has heretofore been proposed a device in which a differentiated output of the metering circuit is used for the correction thereof. Such a device calculates (1) a voltage V', namely, EQU V'=(dV/dt).sub.t=t .multidot..DELTA.t
proportional to the primary differentiation value of the metering output V at each time, in order to detect the speed of variation of the quantity of object light passing through the aperture opening by obtaining a voltage corresponding to the variation with time of the metering output. In the above equation, .DELTA.t is the aforementioned time delay, (2) a level correction value .DELTA.V' which incorporates therein the acceleration of the variation of the quantity of object light, namely, EQU .DELTA.V'=(.DELTA.t.sup.2)
is previously given, where k is constant. The two are added together and when the following equation (1) is established at a certain time t.sub.1, the electromagnet is demagnetized. EQU V.sub.1 =VS+(dV/dt).sub.t=t.sbsb.1 .multidot..DELTA.t+.DELTA.V'(1)
where
V.sub.1 : metering output at time t.sub.1
VS: set voltage by the setting of the shutter speed and film sensitivity
In other words, when stop down is effected, the aperture diameter is gradually reduced from its open position so that the metering output is also gradually reduced. Accordingly, by increasing the set voltage V.sub.s by a voltage V' proportional to the differentiation value of the metering output and the level correction value .DELTA.V', the electromagnet is demagnetized earlier by that value. It is after lapse of the delay time .DELTA.t that the stop down operation is actually stopped after demagnetization of the electromagnet, and the aperture value is controlled when it substantially approaches a proper value.
However, the metering output is slowly varied where the aperture is near the open position and is sharply varied as the aperture is stopped down because (1) the movement of the stop down lever for the objective lens is varied in proportion to the diameter of the aperture opening, namely, the amount of movement of the stop down lever per step of aperture is smaller as the stop down progresses and (2) due to the construction of the stop down lever which is pulled on in one direction by a spring, the movement of the stop down lever after the stop down lever has been released from its restraint is accelerated by the movement thereof until it is stopped by a locking member. Therefore, the variation of the metering output includes an acceleration component even if setting is effected so that equation (1) is established at a certain aperture value as has been done in the prior art, and the term .DELTA.V' in equation (1) is a value which is variable with variation in aperture value, and this has led to a disadvantage that accurate correction cannot be achieved by another aperture value.
Also, in a camera wherein the lens is interchangeable, the relation between the aperture opening of the objective lens and the amount of movement of the stop down lever is not constant because of a large or a small aperture opening for the same aperture value depending on the type of the objective lens (for example, telephoto lens or wide angle lens), there is irregularity in the relation between the metering output and the lapse of time for the stop down.
In a camera exhibiting such a characteristic, if control is effected by the conventional control device described hitherto, substantially proper control may be achieved for one objective lens but the following problem arises in other type of objective lens. If adjustment is effected so that substantially proper control can be achieved by a standard lens under an object light, the metering output is sharply varied in an objective lens having the same aperture value as the standard lens but having a smaller aperture diameter than the standard lens, so that the aperture opening is greatly varied within the delay time .DELTA.t. Therefore, the level correcting output .DELTA.V' must be great and the time for stopping the stop down must be earlier, otherwise there can occur under-exposure resulting from excessive stop down. On the other hand, in an objective lens having the same aperture value as the standard lens but having a greater aperture diameter than the standard lens, the metering output is slowly varied so that the aperture diameter is not so much varied within the delay time .DELTA.t. Therefore, the aforementioned level correcting output .DELTA.V' must be small and the time for stopping the stop down must be delayed, otherwise there can occur over-exposure resulting from insufficient stop down. That is, in the prior art device, the aforementioned level correcting output .DELTA.V' must be adjusted depending on the type of objective lens in use.